Muller cells are believed to play a key role in generated many components of the electroretinogram (ERG). Studies of ERG currents within the retina and the preliminary observations reported here indicate that membrane conductivity in these glial cells is unevenly distributed across the cell surface: most of the Muller cell K+ conductance is localized in the endfoot region of the cell. I propose to study this regional difference in membrane properties by recording intracellularly from Muller cells in perfused, retinal-slice preparations. Membrane specificity for K+ and other ion species will be investigated by monitoring cell membrane potential as the preparation is perfused with solutions containing different concentrations of K+, Na+, and C1-. The membrane properties of localized regions of Muller cells will be studied by monitoring cell responses as K+ solutions are pressure-ejected from an extracellular pipette into different retinal layers. High spatial resolution will be achieved by visualizing individual Muller cells with the intracellular injection of the non-toxic dye Lucifer Yellow (viewed with fluorescence microscopy). This will permit the positioning of the ion ejection pipette immediately adjacent to fine Muller cell processes. Ion ejection experiments will also be conducted in the presence of ouabain to assess the effect of Na+-K+ pump activity on Muller cell K+ responses. A specialization of the Muller cell endfoot region would have important implications for retinal physiology. Elucidating such specifications would clarify the role that the Muller cell plays in generating the ERG, which is an important clinical tool for diagnosing many retinal diseases, including diabetic retinopathy and retinitis pigmentosa. Such specializations would have significant effects on the control of extracellular K+ levels in the retina. The experimental preparation may also prove to be a good model system for studying glial cell control of extracellular K+ levels and the generation of slow potentials within the central nervous system.